1. Field of the Invention
The invention relates to a process for the preparation of tertiary olefins in pure form by cleavage of the appropriate tert.-alkyl alkyl ethers on strongly acidic cation exchangers in the H.sup.+ form.
2. Background Information
Tertiary olefins are important precursors for oligomers, for example, for solvens and lubricants, for polymers and copolymers, and for higher grade chemicals, such as pinacolin, neocarboxylic acids, isoprene and others.
Tertiary olefins are produced in crude form, for example, in the thermal and catalytic cracking of light petroleum, naphtha and other suitable starting materials, or on dehydrogenation and/or isomerization thereof; they usually exist here as a mixture of a large number of saturated and unsaturated attendant materials whose distillative separation is difficult and expensive since it starts from distillation cuts which contain substances of similar boiling point and with the same or a similar number of C atoms. The tertiary olefins are thus generally isolated via selective reaction, separation of the reaction product and decomposition of the separated pure reaction products. Whereas the selective esterification using sulphuric acid and decomposition of the esters formed were used previously, in recent times the selected esterification of the tertiary olefins using alkanols on acidic cation exchangers is preferred in order to avoid using corrosive sulphuric acid. The tert.-alkyl ethers formed may be separated off from the materials accompanying the tertiary olefins by known methods (distillation, azeotropic distillation, extractive distillation, inter alia) and obtained in pure form. The tert.-alkyl alkyl ethers may then be cleaved into the basic tertiary olefins and alkanols; the separation from the alkanols and remaining ether of the tertiary olefins thus prepared presents no difficulties. The cleavage of the ethers is carried out on suitable catalysts at temperatures which are higher than the temperatures required for the formation of the ethers.
Mainly mineral catalysts, such as silicic acids or aluminium oxides having large surface areas, silicoaluminates, mordenites, zeolites, oxides of other elements, phosphoric acid or salts which react in an acidic fashion, were hither-to used as catalysts for the ether cleavage. The mineral catalysts are used at higher temperatures up to 673K. However, undesired by-products, particularly dialkyl ethers, are produced during this from the alkanols produced during the cleavage. This formation of dialkyl ethers increases with increasing temperature and thus removes the basic alkanol, which should be recycled in the overall esterification/ether cleavage process, from the cycle and additionally complicates the work-up. On the other hand, a temperature increase must generally be carried out in order to activate the catalyst sufficiently and in order to achieve sufficient supply of heat for the endothermic ether cleavage. On the other hand, the danger of hydration of the tertiary olefin to be obtained becomes greater due to the water produced during the ether formation, losses of this desired tertiary olefin arising in addition to the losses of alkanol or a downstream hydration necessarily being provided.
In spite of the danger of hydration of the tertiary olefins, the cleavage of ether has been carried out in the presence of water in order to control the undesired formation, occurring primarily, of the dialkyl ethers GB 1,176,620; DE-OS (German Published Specification) 3,142,461). This addition of water also presented itself since the likewise undesired hydration of the tertiary olefins does not yet come to the fore too much at the comparatively high reaction temperatures necessary for the use of the mineral catalysts.
However, acidic cation exchangers in the H.sup.+ form have also already been proposed for the cleavage of the tert.-alkyl alkyl ethers (DE-AS (German Published Specification) 1,216,856; U.S. Pat. No. 4,447,668). In this case, reaction temperatures of 333.degree.-393.degree. K. are used. However, the undesired dialkyl ethers can not be avoided as by-products on acid cation exchangers either; further undesired by-products are the oligomers of the tertiary olefins produced. If the reaction temperature is increased in order to increase the conversion of the tert.-alkyl alkyl esters to be cleaved, the formation of the by-products mentioned also increases. On acidic cation exchangers therefore, either relatively low selectivities have been achieved at relatively high temperatures or relatively low conversions have been achieved at relatively low temperatures.